The present invention relates generally to platform scales, and more particularly to vehicle platform scales having strain gauge load sensors affixed to deflectable load cells.
Platform scales for measuring axle weight loads of motor vehicles are known in the prior art to include generally one or more elongate weighbridge platforms mounted on load cell assemblies. Such platform scales are generally employed in pairs positioned parallel to one another and spaced apart to accommodate the left and right-hand sets of wheels of a vehicle driven onto the scales. Each weighbridge platform is sufficiently wide to accommodate the wheel or wheels at one end of an axle. Each platform scale normally includes at least two weighbridge platforms aligned end-to-end in order to accommodate the multiple wheels of a dual tandem rear axle assembly of a conventional commercial truck. Longer platform scales having three or more weighbridge platforms are also available to accommodate triaxle trucks or special purpose vehicles.
In operation, a vehicle is driven onto a pair of parallel, spaced platform scales and stopped momentarily with a set of axles centered on the scales. The weight loads on the individual weighbridge platforms are measured and electronically combined to indicate the total weight load of the axle at a remote operator's station. The weight loads of successive sets of axles are sequentially recorded and summed after all axle assemblies have been weighed to give the gross weight of the vehicle.
The weight load on each individual weighbridge platform is typically borne entirely by two load cell assemblies that support the opposite ends of the platform. Each load cell assembly includes a deflectable load cell positioned transversely with respect to the longitudinal axis of the weighbridge platform. The load cell typically consists of a rigid steel beam supported at its opposite ends in a manner such that its midsection may deflect downwardly in response to a weight load from the weighbridge platform.
The load cell typically includes a number of strain transducers, generally referred to as strain gauges, affixed to its lower surface. Downward deflection of the load cell is manifested by tensile stresses in the lower surface of the load cell. The strain gauges respond to such stresses by producing an electrical signal that varies in magnitude with the extent of the load cell deflection. The load cell and its strain gauges are calibrated such that the assembly can be used to accurately and reproducibly measure weight loads on the load cell. In operation of the platform scale, the electrical outputs from the two load cells under the opposite ends of a weighbridge platform are combined electronically to obtain a measure of the total weight load on the platform.
Ongoing efforts have been made to improve the durability, accuracy and precision of vehicle platform scales of the type thus far described. Such efforts have been addressed both to the general object of improving the performance of load cell assemblies in all applications, as well as to the more specific object of improving the performance of vehicle platform scales intended for routine use at truck stops and truck inspection stations.
More specifically, it has been sought to develop a load cell assembly having a floating load cell, that is, a load cell assembly wherein the load cell is freely cantilevered off a pair of end supports and wherein the deflection of the load cell is not restricted by mounting or securing devices normally employed to maintain the load cell in its proper position. Ideally, a load cell should be freely cantilevered off of its underlying end supports such that its midsection may deflect freely downwardly under the effect of a weight load. The ends of the load cell should be free and unconstrained, as by fastening bolts or mounting devices securing the load cell in place. Even though the ends of the load cell move only a slight amount during deflection, it is found that fastening of the ends of the load cell to the underlying end supports or to the frame of the load cell assembly imposes stresses on the load cell during deflection which significantly affect the electrical outputs of the strain gauges. Nevertheless, it is also necessary that the position of the load cell be fixed with respect to the load cell end supports and with respect to the overlying weighbridge platform, since the electrical outputs of the strain gauges are significantly affected by the position at which the weight load is imposed on the upper surface of the load cell and also by the positions of the end supports under the load cell. Large variations in the outputs of the strain gauges result if the load cell is allowed to creep or otherwise move about on stationary end supports, or if the point or points at which the weight load is imposed on the load cell is allowed to vary. Prior art platform scale assemblies have employed fastening devices designed to bind the ends of the load cell as little as possible, but which nevertheless affect the outputs of the strain gauges to some extent.
For example, in certain platform scales developed by the applicants, a load cell was mounted on cylindrical bearing rods which rested in cylindrically curved grooves in underlying bearing plates. The bearing rods were thus retained in fixed positions by the grooves. The bottom surfaces of the load cell were flat where they bore upon the bearing rods in order that the ends of the load cell would be freely cantilivered off the bearing rods and undergo a slight rolling motion on the bearing rods during downward deflection of the load cell under a weight load. In order to keep the load cell precisely located on the bearing rods, it was necessary to bolt the ends of the load cell to the underlying bearing plates and the frame of the load cell assembly. As a result, a portion of weight loads imposed on the load cell was borne by the bolts securing the ends of the load cell in position, as opposed to the load being borne entirely by the deflectable load cell. Although the load cell assembly was capable of being calibrated to give accurate weight measurements, it was found that the calibration was difficult to maintain due to hysteresis of the securing bolts and shifting of the bolts and the load cell. Frequent recalibration of the scale and adjustment of the torque of the retaining bolts was necessary to ensure accurate operation of the scale.
Certain other performance requirements are unique to vehicle platform scales intended for routine commercial use in truck stops or for truck inspection purposes by law enforcement officers. For example, such scales must be sufficiently rugged to withstand hard braking or skidding of vehicles on the scales. Also the scales must be able to withstand vehicles occasionally being driven onto the scales improperly, for example at an angle, as well as vehicles being positioned off center on the weighbridge platforms. The scales must also operate accurately and reliably under all weather conditions, and must be protected against the effects of snow and ice accumulating under and around the weighbridge platforms and the load cell assemblies.
Portable vehicle platform scales have other types of performance requirements. Portable scales are used, for example, by law enforcement officers and regulatory agencies in the establishment of temporary vehicle inspection stations. Such scales must be easy to handle and transport and yet must maintain their calibration during handling and moving. Moreover, portable scales are typically used on a flat roadway where they cannot be embedded in a recess in the pavement to form a street-level scale in the customary manner of a permanent scale facility. Accordingly, portable scales must be constructed as low as possible to the ground and must include entry and exit ramps leading to and from the opposite ends of the platform. It is desirable to have such scales as low as possible to the ground for ease of operation and particularly to ensure accuracy in the weighing of tank trucks. Tank trucks undergo a shift in their center of gravity due to shifting of fluid when one set of axles is raised relative to the others. A rise in the height of one axle, even by a few inches, may cause a significant inaccuracy in the resulting gross weight.
Accordingly, it is a general object of the present invention to provide an improved platform scale.
More specifically, it is an object of the present invention to provide a platform scale with a load cell assembly having a floating load cell.
It is also an object to provide a load cell assembly wherein a load cell is freely cantilevered off a pair of load cell supports, yet wherein the load cell is also maintained in a fixed, predetermined position with respect to its underlying end supports and with respect to the overlying weighbridge platform.
It is another object of the present invention to attain the foregoing objects and also provide an improved low profile, portable vehicle scale that is unaffected by severe weather conditions and which is sufficiently durable to withstand routine handling and moving from one location to another.